1. Field of the Invention
This invention relates generally to a semiconductor device and, more particularly, to a semiconductor device for use in bootstrap circuits included in an Erasable and Programmable Read-Only Memory (hereinafter referred to as EPROM) device which can be erased by ultraviolet light and reprogrammed bit by bit with appropriate voltage pulses.
In general, an EPROM is provided with a means for producing a high-voltage and high-power signal when the EPROM is reprogrammed. One of such means is a bootstrap circuit. A package for accommodating a chip on which an EPROM is formed has a window for receiving ultraviolet light. When it is necessary to erase the content of the EPROM, the entire chip is exposed to strong ultraviolet light from an ultraviolet light generating unit. The ultraviolet light comes through the window of the package accommodating the chip. However, when the entire package is put into a box, the package is not exposed to any kind of light. In addition, the EPROM can be used in a light room, since the intensity of the ultraviolet component of the light in the light room is so small that the EPROM would not be erased.
2. The Prior Art
A prior art semiconductor device for use in bootstrap circuits included in an EPROM which can be erased by ultraviolet light comprises a capacitor, wherein the difference in potential between the two conducting surfaces of the capacitor causes a high-voltage and high-power signal. In order to reprogram the EPROM effectively, the difference potential should be maintained at a high level as long as possible.
However, in the prior art device, when the EPROM is reprogrammed in a light room, a connection node of the capacitor and a MIS (Metal-Insulator-Semiconductor) field effect transistor are exposed to light. The light penetrates into a p-n junction portion formed by a substrate of one conductivity type and the source or drain region of another conductivity type of the MIS field effect transistor. As a result, the charge of the source or drain region is decreased and, accordingly, the charge of the capacitor is also decreased. This is because one of the conducting surfaces of the capacitor is connected to the source or grain region through one or more electrically conductive materials. Therefore, the difference in potential of the capacitor becomes small, and this may cause the writing of new data into the EPROM in a light room to fail.